By studying the Hadza’s lifestyle, Pontzer thought he would find evidence to back the conventional wisdom about why obesity has become such a big problem worldwide. Many have argued that one of the reasons we’ve collectively put on so much weight over the past 50 years is that we’re much less active than our ancestors.

Surely, Ponzer thought, the Hadza would be burning lots more calories on average than today’s typical Westerner; surely they’d show how sluggish our bodies have become.

On several trips in 2009 and 2010, he and his colleagues headed into the middle of the savanna, packing up a Land Rover with camping supplies, computers, solar panels, liquid nitrogen to freeze urine samples, and respirometry units to measure respiration.

In the dry, open terrain, they found study subjects among several Hadza families. For 11 days, they tracked the movements and energy burn of 13 men and 17 women ages 18 to 75, using a technique called doubly-labeled water — the best known way to measure the carbon dioxide we expel as we burn energy.

When they crunched the numbers, the results were astonishing.

“We were really surprised when the energy expenditure among the Hadza was no higher than it is for people in the US and Europe,” says Pontzer, who published the findings in 2012 in the journal PLoS One. While the hunter-gatherers were physically active and lean, they actually burned the same amount of calories every day as the average American or European, even after the researchers controlled for body size.